Evolution of a Carboniferous carbonate-hosted sphalerite breccia deposit, Isle of Man

A newly discovered, extensive sphalerite-bearing breccia (similar to 7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician-Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T (h) > 300A degrees C), high-salinity (20-45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II-IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of similar to 105-180A degrees C and salinities of similar to 15-20 wt.% equiv. NaCl. The delta S-34 values of sphalerite (6.5-6.9aEuro degrees Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The delta O-18 values for quartz and dolomite indicate two main fluid sources in the breccia's hydrothermal system, local Carboniferous-hosted brines (similar to 0.5-6.0aEuro degrees Vienna standard mean ocean water) and basement-involved fluids (similar to 5.5-11.5aEuro degrees). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.